The present invention relates to an improved process for the removal of urea, ammonia, and carbon dioxide from dilute aqueous solutions obtained as process condensate from coupled ammonia and urea syntheses.
In the preparation of ammonia from hydrocarbons, primarily methane, a synthesis gas is first prepared in a primary reformer by reacting the hydrocarbon with an excess of steam, generally according to the reaction equation CH.sub.4 +H.sub.2 O.fwdarw.CO+H.sub.2. Subsequently, the gas mixture leaving the primary reformer is introduced into a secondary reformer together with air in order to further convert any hydrocarbon present with oxygen, and, moreover, to introduce the nitrogen required for the ammonia synthesis.
The gas mixture leaving the secondary reformer mainly consists of water vapor, carbon dioxide, nitrogen, carbon monoxide, and hydrogen. This mixture is treated catalytically to convert the carbon monoxide, generally according to the reaction equation CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2, so that a gas mixture of carbon dioxide, nitrogen, hydrogen, water vapor, and small quantities of carbon monoxide is obtained. The excess water vapor in this mixture is condensed and forms the so-called process condensate. The carbon dioxide is removed from this gas mixture by washing, and the small quantity of carbon monoxide remaining is converted in a methanator into methane, which is not poisonous to the ammonia synthesis catalyst. The resulting gas mixture containing primarily nitrogen and hydrogen is a suitable starting mixture for the ammonia synthesis.
During the various processing steps, small quantities of by-products are also formed, such as ammonia in the secondary reformer, and methanol, methylamine, and other organic impurities in the carbon monoxide conversion. When the excess water vapor is condensed, these by-products will enter the process condensate. The process condensate additionally will contain dissolved carbon dioxide and traces of metal compounds originating from the catalysts and the equipment.
The presence of these impurities in the process condensate is undesirable, whether the process condensate is to be fed back into the system as boiler feed water, or whether it is to be discharged to the environment as waste water.
The quantity of process condensate resulting from an ammonia plant depends on the steam/hydrocarbon ratio in the primary reformer and, when considered together with the small quantities of condensate from the coolers of the synthesis gas compressor, and condensate obtained in the methanator, amounts to about 1.1 to 1.25 tons per ton of ammonia produced. This condensate as a rule contains about 0.08 to 0.1 percent by weight ammonia, 0.15 to 0.2 percent by weight carbon dioxide, 0.1 to 0.2 percent by weight methanol, 30 to 50 ppm organic impurities and traces of the metals iron, copper, zinc, aluminum, sodium, and calcium.
In the preparation of urea from ammonia and carbon dioxide, a urea synthesis solution, still containing a substantial quantity of free ammonia and non-converted ammonium carbamate, is formed at elevated temperature and pressure. The carbamate is thereafter decomposed in one or more steps into ammonia and carbon dioxide. This ammonia and carbon dioxide are driven out of the urea solution together with the free ammonia present, and are usually recirculated to the urea synthesis reactor. In the final carbamate decomposition step, the aqueous urea solution obtained still contains some quantities of dissolved ammonia and carbon dioxide, and these are substantially removed by expansion to atmospheric or lower pressure. The resulting aqueous urea solution is concentrated by evaporation and/or crystallization and further processed.
During this evaporation process, a gas mixture is formed containing, in addition to water vapor, ammonia and carbon dioxide, together with entrained fine droplets of urea. This gas mixture, as well as the gas mixture separated off during the expansion of the urea solution after the final decomposition step, is condensed to form process condensate. The process condensate thus obtained is in part fed back into the urea process to absorb the gas mixture separated out in the final decomposition step. The remaining portion of this condensate is generally discharged from the process.
The process condensate from a urea synthesis plant also includes the water fed into the process as steam for operating the ejectors in the evaporation and/or crystallization section, washing water, flushing water to the stuffing boxes of the carbamate pumps, and the like. In addition, one mole of water is formed for each mole of urea produced. Thus, in a urea plant having a capacity of 1900 tons of urea per day, 570 tons of water is formed, and in addition thereto, depending on the temperature of the cooling water used in the process, about 380-600 tons of water per day is fed into the process, so that approximately 950-1170 tons of water in total must be removed from the process per day.
This process condensate resulting from a urea synthesis process generally contains about 2 to 9 percent by weight ammonia, 0.8 to 6 percent by weight carbon dioxide, and 0.3 to 1.5 percent by weight urea. To simply discharge these materials from the process with the process condensate represents, on one hand, a loss of substantial quantity of raw materials. On the other hand, this represents a substantial load to the surface waters into which this waste water would be discharged, and is no longer permitted in many countries.
Consequently, in coupled installations for the preparation of both ammonia and urea, large quantities of process condensate are formed in each installation, which vary greatly in composition, and which can be discharged to the environment, or used as boiler feed water, only after careful purification.
In Industrial Wastes, September/October 1976, pages 44-47, a process is described, wherein process condensate obtained in a urea synthesis plant, which has already been freed from a portion of its ammonia and carbon dioxide at a low pressure, is passed, at a higher pressure, into the bottom of a reaction column. In the reaction column it is heated by steam so as to hydrolyze the urea present. Thereafter, the resulting solution of reduced urea content is expanded and stripped with steam in a desorption zone. This method, wherein the solution being processed flows cocurrently with the steam and the gaseous ammonia and carbon dioxide, has the drawback that even after a long residence time of the solution in the reaction column, the final urea and ammonia content is relatively high.
It has previously been suggested in Netherlands patent application No. 7705356 that superheated steam be formed from the process condensates obtained from ammonia and urea synthesis processes. To accomplish this, the process condensate obtained in the urea preparation is first fed into a hydrolyzing column wherein the urea is hydrolyzed. The ammonia and carbon dioxide formed by the hydrolysis are then desorbed in a desorption column, and the remaining solution, together with the process condensate obtained from the ammonia preparation, is converted into superheated steam. This superheated steam is then fed to the primary reformer in the ammonia synthesis process. Optionally, some of the ammonia and carbon dioxide can be removed from the combined process condensate by expansion and stripping off the gases liberated in this process, prior to converting the condensate into superheated steam.
The disadvantage of the above-described process is that the presence of ammonia near equilibrium conditions impedes the hyrolysis of the urea such that the final concentration of urea is higher than it would be in the absence of ammonia. This incomplete hydrolysis of urea presents a real danger in that, when urea-containing process condensate is converted into steam, the urea in the steam boiler will decompose into ammonia and carbon dioxide and present a significant risk of corrosion.